1. Field of the Invention
The present invention relates to an optical pickup apparatus which can compatibly read, with one laser diode conventionally adapted for compact disks (center wavelength: 780 nm) only, information recorded in any of two types of recording media having respective recording densities different from each other.
2. Description of the Related Art
Currently, in information input/output apparatuses using light, such as a compact disk drive, a recording pit is formed by converging light emitted from a laser beam source as a micro spot on a track provided on a disk-shaped recording medium such as a compact disk (CD). The presence or absence of such a pit is recorded as information, and then the micro spot on the track is irradiated with light from the laser beam source to detect the presence or absence of the pit on the track by reflected light, thereby reading the information.
Recently, digital video disks (DVDs) having a recording capacity of about 7 times as large as that of CDs have been widely used to meet the demand for increased recording capacity. An increase in recording capacity involves improvement in recording density, which depends upon the number of recording pits that can be formed on a recording medium (hereinafter, referred to as an optical disk). In DVDs, decreasing the size of the recording pit, that is, decreasing the diameter of the spot of a light beam irradiated on the optical disk is one of factors in increasing the density. The size of the spot to be irradiated on the optical disk is proportional to the wavelength of the laser and is inversely proportional to the numerical aperture of an objective lens. Accordingly, for decreasing the size of the recording pit, it is required to decrease the wavelength of the laser and to increase the numerical aperture of the objective lens.
DVDs are strongly required to be compatible with CDs. Originally, an optical head device was equipped with one laser beam source having a wavelength of 635 to 650 nm and one objective lens having a numerical aperture of about 0.6 for DVDs, and also with another laser beam source having a wavelength of 780 nm and another objective lens having a numerical aperture of about 0.45 for CDs so as to ensure compatibility between DVDs and CDs.
However, when the numerical aperture of the objective lens increases, the convergence of light beam deteriorates due to coma aberration with respect to the tilt of the optical disk. Since coma aberration is proportional to the cube of the numerical aperture of the objective lens and to the thickness of the optical disk substrate, DVDs are designed to have a substrate thickness of 0.6 mm, which is equal to the half of CDs' substrate thickness.
When the thickness of the substrate deviates from a designed value, spherical aberration will occur at a place where light passing through the central portion of the objective lens and light passing through the outward portion thereof converge. Therefore, when a CD is read by the use of an objective lens having a numerical aperture optimally designed for a DVD's substrate thickness, it is necessary to correct spherical aberration by limiting the outermost flux of light incident on the objective lens or by slightly diverging the incident angle at the lens.
Accordingly, while one objective lens can work for both DVDs and CDs with the necessary correction of spherical aberration, two laser beam sources having respective wavelengths as described above have to be provided for compatibility with write-once-read-many CDs. This is because a reflective recording layer of write-once-read-many CDs is formed of an organic dye material and thus has a reflection coefficient as low as 6% for a light beam having a wavelength of 635 to 650 nm, that is a wavelength appropriate to DVDs.
Thus, since the current DVD optical head apparatuses are equipped with two laser beam sources respectively with a wavelength of 635 to 650 nm for DVDs and a wavelength of 780 nm for CDs, and since respective light beams emitted from the two light sources are to be guided to one objective lenses thereby requiring additional parts such as a prism, an aperture control device, or the like, it is very difficult to reduce the size and cost of the apparatus.
FIGS. 15A and 15B show an embodiment of a conventional optical pickup apparatus. FIG. 15A is for reading a high-density optical disk, as typified by a DVD, and FIG. 15B is for reading a standard-density optical disk, as typified by a CD.
As shown in FIG. 15A, a laser beam source 2 with a wavelength of 635 to 650 nm is used in order to support a high-density optical disk 1. First light emitted from the laser beam source 2 passes through a wavelength-selection prism 15 and a beam splitter 9, and is converted into parallel pencil by a collimating lens 14. The light in parallel pencil is incident on a first light-selection device 3 at an angle of 45°, and is totally reflected at a flat first reflecting surface 4 in a direction perpendicular to an objective lens 6. Then, the light is incident on the objective lens 6 whose numerical aperture is set for a high-density optical disk, and is converged onto a recording surface of the high-density optical disk 1 by the objective lens 6. The first reflecting surface 4 is flat and composed of a wavelength-selection film 30 formed of a dielectric multi-layer film.
On the other hand, as shown in FIG. 15B, a laser beam source 12 with a wavelength of 780 nm is used in order to support a standard-density optical disk 11. Second light emitted from the laser beam source 12 is reflected by a wavelength-selection prism 15 at an angle of 90°. Then, the reflected light is incident on the first reflecting surface 4 of the first light-selection device 3 at an angle of 45°, in a manner similar to the first light. At this time, the incident light passes through the first reflecting surface 4, and is reflected at a second reflecting surface 7. The second reflecting surface 7 is concaved, and a metal film 8 as a total-reflection film is formed thereon at the central circular area thereby controlling the beam diameter of the light coming through the flat surface. The other area of the concaved surface than the circular total-reflection film is covered by an anti-reflection film 16 so that light that is not reflected by the total-reflection film does not get astray.
The shape (not shown) of the metal film 8 is designed to be a substantially perfect circle seen from the optical axis, and its outside diameter is set such that light, when converged onto the standard-density optical disk 11 by the objective lens 6, produces an optimum beam diameter. Such a function of controlling the beam diameter of light incident on the objective lens 6 is called an aperture control function. The light coming through the flat surface side is totally reflected only at the metal film 8, which constitutes the second reflecting surface 7 on the concaved surface side, directed toward the objective lens 6, made incident thereon, and converged onto the standard-density optical disk 11. At this time, since the beam diameter is controlled by the aperture control function, the light is optimally converged onto the recording surface of the standard-density optical disk 11 by the objective lens 6.
As described above, in the conventional optical pickup apparatuses, spherical aberration, which is caused by the fact that the high-density optical disk substrate 1 has a thickness of 0.6 mm, whereas the standard-density optical disk substrate 11 has a thickness of 1.2 mm, is corrected, thereby converging light onto the recording surface of the standard-density optical disk 11. Specifically, the light is slightly diverged at the metal film 8 serving as the second reflecting surface 7 having curvature, thereby being converted into a pencil of rays slightly diverging to the objective lens 6. The direction of focusing is controlled by adjusting an actuator 17. The light reflected at the recording surface of either optical disk takes the incoming route backward, is made incident on a beam splitter 9 and directed thereby toward a cylindrical lens 10 to reach a photodetector 13, where the light is detected and gets the intensity of its signal light converted into an electrical signal, whereby recorded information is read.
Another embodiment of a conventional optical pickup apparatus shown in FIGS. 16A and 16B is disclosed in Japanese Patents Laid-open No. 10-241193 and No. 11-3528 by the inventors of the present invention, in which one laser beam source can support both a standard-density optical disk and a high-density optical disk. As shown in FIGS. 16A and 16B, the optical pickup apparatus uses only one laser beam source 20 with a wavelength of 635 to 650 nm for both a standard-density optical disk 11 and a high-density optical disk 1. Light emitted from the laser beam source 20 is transmitted with or without its polarization plane being rotated by a polarization plane rotating device 21. The light, when incident on a polarizing plate 23 provided ahead of an objective lens 22, is either blocked (FIG. 16B) by a filter portion 23A or passes (FIG. 16A) therethrough depending on the angle of the polarization plane.
Thus, the polarization plane rotating device 21 and the polarizing plate 23 enables the objective lens 22 to control a numerical aperture, thereby varying the diameter of the beam converged by the objective lens 22. The rotation of the polarization plane by the polarization plane rotating device 21 can be controlled by turning on and off a switch SW of a control circuit A thereby applying thereto a signal from the circuit A.
Since the optical pickup apparatus of FIGS. 15A and 15B needs two types of laser beam sources having different wavelengths from each other for a standard-density optical disk and a high-density optical disk, respectively, problems are involved in reducing cost and downsizing. Moreover, since the plano-concave lens serving as a reflection mirror has a function of controlling the aperture, three types of films: a wavelength filtering film; a total-reflection film; and an anti-reflection film must be formed, thus increasing cost and also giving technical difficulty.
The optical pickup apparatus of FIGS. 16A and 16B uses only one laser beam source with a wavelength of 635 to 650 nm for reading a high-density optical disk. When reading a standard-density optical disk, the polarizing plate 23, which is provided ahead of the objective lens 22 and adapted to block the periphery of light, transmits only the central portion of the laser beam by means of a polarizing filter, and light is converged to an appropriate spot diameter onto a recording surface.
However, as described above, the thickness of a substrate is different between the high-density optical, disk and the standard-density optical disk. Accordingly, the problem that spherical aberration occurs in a beam spot converged on the recording surface of the standard-density optical disk 11 cannot be solved, adversely affecting reading of recorded information. Furthermore, laser beam sources with a wavelength of 635 to 650 nm are more expensive than those with a wavelength of 780 nm failing to realize a substantial cost reduction, and also inferior thereto in reliability and output characteristics.